Sprowl you yourself just nailed it. There is some data (velocity), scouting reports and a lot of hype. We heard all this about Dice K how he was a surefire front line starter. Heck wasn't Tazawa originally reported to have a 94-95 mph fastball? Time will tell.
Tazawa's claim to fame was always his fastball command with average velocity at best. By the time we saw him in the majors, he was within a few months of Tommy John surgery and barely touched 90 mph. When he returned last summer, he threw 91-93, so there is a reasonable prospect that his velocity is back to average, but nobody had Tazawa in Matsuzaka's league, much less Darvish's.
Where Darvish is concerned, we have a lot more than just scouting reports. He has pitched in international competitions for several years. His videos are all over Youtube, so viewers can check out his repertoire and movement for themselves. The Rangers have themselves a very strong starting rotation.